If you've been trying to send mail to Apple's iCloud email domains today (mac.com, me.com, icloud.com), you might have run into this bounce message:
5.1.1 (bad destination mailbox address) <email address> : user lookup success but no user record found
It's not just you -- Apple appears to have had intermittent issues today. Their status page lists it as now resolved, but for some period of time, your attempts to mail a valid recipient might have been met with the bounce message seen above.
[ H/T: Andrew Barrett ]